DESCRIPTION: Distinct from the common impression, the genomes of individual cells in our body are not exactly homogenous or static. Heterogeneity and dynamic changes occur in many cases, such as aging, cancer and HIV infection. Detecting these single cell events in a live animal remains a major challenge. In the proposed project, we plan to validate and further develop our recently invented CRISPR imaging technique in a multicellular organism. This technique utilizes the CRISPR/Cas9 system to fluorescently label specific, endogenous genomic loci for microscopy detection. Here, we will use the nematode C. elegans as the model organism to validate and benchmark CRISPR imaging as a technology to measure the size of repetitive genomic elements such as telomeres as well as the copy number of genes. We will also improve the sensitivity of CRISPR imaging for the detection of non-repetitive genomic elements. Finally, we will apply CRISPR imaging to the study of telomere length change at the single cell level during C. elegans development and aging.